This invention relates to the control of radiant burners. Also known as surface combustion, radiant energy or infrared burners, radiant burners are used in various types of heating appliances. More particularly, the invention relates to a method and apparatus for setting and maintaining the proportion of fuel gas to air in the combustible gas mixture supplied to a radiant burner at an optimum value.
Under ideal conditions, a radiant burner would burn with highest thermal efficiency and lowest production of undesirable emissions when the combustible gas supplied to the burner is a stoichiometric mixture of fuel gas and air, i.e. when the amount of air supplied is exactly sufficient to completely oxidize the amount of fuel supplied. Should the ratio of fuel to air increase above the stoichiometric value, or the mixture becomes fuel rich, however, unburned fuel and carbon monoxide will be present in the combustion gases produced by the burner.
Under actual operating conditions, if a radiant burner were to be configured to operate exactly at the stoichiometric ratio, design or manufacturing defects, transient or chronic departures toward the fuel rich condition from the stoichiometric ratio either generally or locally on the burner surface can result in the production of undesirable and hazardous emissions from the burner. It is general design and engineering practice therefore to operate radiant burners with the fuel air mixture containing some amount of excess air, i.e. where the combustible gas is fuel lean or the fuel to air ratio is below the stoichiometric ratio. Operating in an excess air condition helps to assure that all fuel will be burned and no hazardous combustion products formed. The optimum amount of excess air necessary in a given burner installation depends on a number of factors such as the construction and geometry of the burner and its surroundings as well as the type and composition of the fuel to be burned. In general, the typical radiant burner will begin to exhibit undesirable combustion characteristics as excess air decreases to less than about five to ten percent. In such a burner installation, it is common to design for an excess in percentage in the range of 15-30 percent. Operation at excess air percentages greater than within that optimum range results in degradation of burner performance, loss of efficiency or blowout.
While it is possible to directly measure the flow ratio of the fuel gas and air supplies to a burner and to regulate one or both of the flows so as to produce a combustible gas mixture that is optimum, such a detection and control system would be complex and prohibitively expensive in many applications. The designs of some burner applications include pressure switches to detect air flow rate, but such switches are capable only of detecting gross departures from the optimum excess air value and not of regulating the excess air percentage. Still other designs employ sensors which detect the presence and concentration of constituents, such as oxygen, of the flue gases emanating from the burner. Those designs however are subject to sensor fouling and can be unreliable and inaccurate. What is needed therefore is an economical, accurate and dependable means to automatically ensure that a radiant burner is supplied with a combustible gas that contains the optimum amount of excess air.